My opinion on all the members:
Gandalf: Very important member - was a guide and protector (the fellowship would not have survived Moria without him), and he was instrumental in discovering Saruman's treachery. He also united Rohan by reviving Theoden, of which only he was capable.
Aragorn: Also very important member - defended the hobbits until they reached Rivendell. Otherwise, not so amazingly important to the quest's success.
Sam: Frodo could not have completed the quest without him. sam gave much of his food and water to Frodo towards the end, and saved him from the orcs in Cirith Ungol, as well as Shelob.
Frodo: Had the endurance to complete the quest - otherwise not that important. I personally think that almost anyone in the fellowship could have completed the quest with all the support he was getting.
Merry, Pippin: Helped rouse the ents to go to war against Saruman.
Gimli, Legolas, Boromir: Purely warriors in the quest.